Message from syslogd@cheeky-badger at Dec 20 16:38:20 ...
kernel:[ 2707.000027] [Hardware Error]: Run the message through 'mcelog --ascii' to decode.

it also appears in a terminal if I have one open, I have done a dist-update and am running the latest kernel. It doesn't seem to be affecting anything, but it is annoying having this message popping up all the time.

http://en.wikipedia.org/wiki/Machine_Check_Exception offers some clues about what it might be. Installing mcelog might get you more useful errors. Maybe try running memtest. If you are overclocking try going back to standard settings. Have a look in your logs (syslog and kern.log anyway) for any other clues.